Fun fact: the US Midwest grows corn, which produces as much oxygen (absorbs as much CO2) as the Amazon Basin. During the summer anyway. So we're growing a lot of plants already.
Most of the oxygen we breathe comes from plankton. But we don't particularly care for or about the ocean -- we don't see it, and we can make money by ignoring its health.
It's the American Way (tm)!
And most of that corn goes to feed cattle, which produce methane (worse than CO2). So, what we do with the plants matters. Do we allow them to sequester the carbon or we put their carbon right back into the atmosphere?
Under 40% these days. In Iowa the largest buyer of corn is Pepsi. Oh! Unless by 'cattle' you meant people?